understrapper

[…] I have seen him lately at Boulogne, and am perfectly well acquainted with some persons who have supplied him with French lace and embroidery; and, as a proof of what I allege, I desire you will order him and this barber, who is his understrapper, to be examined on the spot.

Alfie was familiar with the term ‘understrapper’. The Harry Fielding Alfie knew was an understrapper; a bob-a-job man for MI5. Somebody who could do a bit of house- or office-breaking, or some surveillance work.